That last bit is the elephant in the room. US wants to have a global empire and this means being able to project force anywhere across the world. This requires far more resources than simply being able to defend yourself or even project power in a particular region. US is forced to spread their resources ever thinner, while their adversaries keep getting stronger. A great example of this problem has been recently seen with Russia backing DPRK which pins a ton of US resources in the occupied Korea.